Remotely operated vehicles (ROVs) which have been used in the past are of a type wherein a deployment cage is suspended from a surface vessel, and the vehicle is teathered to the cage, the vehicle being of substantially neutral buoyancy and therefore able to adjust its own vertical position by slightly altering its buoyancy.
With such prior art arrangements, the lifting capacity of the ROV is determined by the degree of positive buoyancy which it can attain, and therefore lifting capacity is usually relatively low. Further, since at least a part of the weight of the teather between the ROV and the deployment cage must be borne by the ROV, it is necessary to use a relatively light teather cable and as this cable incorporates all of the electrical wiring between the ROV and the deployment cage, breakages of signal wiring often occur. This problem is accentuated by continual flexing of the teather due to the heaving motion of the deployment cage, which is caused by the motion of the surface vessel in response to swell and chop.
Another problem which can be experienced with ROVs is that as the amount of instrumentation in the ROV is increased, the size of the teather and umbilical cables increase due to the requirement for more wiring to carry signals back and forth between the ROV and remote surveillance and control panels in the surface vessel. One known method of overcoming this problem is to use signal multiplexers and demultiplexers to reduce the number of signal wires required, however, such equipment is relatively complex and expensive and in the event of equipment failure can lead to long down times and costly spare parts inventories.